The present invention relates to an ATM interface apparatus for time-division multiplex highways which is used to connect communication lines in the asynchronous transfer mode (ATM) to existing time-division multiplex highways and, more particularly, to an ATM interface apparatus for time-division multiplex highways which is designed to improve the line utilization efficiency of time-division multiplex highways.
In performing communication in the asynchronous transfer mode, information is transmitted in units of relatively small packets called cells. In ATM communication, the band can be arbitrarily changed, even during communication, from a band for low-speed communication or communication of a small amount of information to a high-speed, broad band. In performing high-speed, broad-band communication, many cells are transmitted. In performing low-speed communication, the cell transmission cycle is prolonged to transmit a small number of cells. In this manner, in ATM communication, the traffic in a communication line changes.
When such a line for ATM communication is to be connected to a conventional time-division multiplex highway, time slots which can cope with the maximum traffic of the ATM line are allocated in a fixed manner.
An ATM interface apparatus is disclosed in Japanese Patent Laid-Open No. 4-68730. In this apparatus, band compression is performed in accordance with the kind of information to be transmitted through cells, and time slots used for a time-division multiplex line in accordance with a band required after compression are allocated. For example, different band compression schemes are used depending on whether the kind of information is "speech" or "digital data".
Japanese Patent Laid-Open No. 2-29196 discloses a technique of allocating a required number of time slots when a call is generated. In this conventional technique, a list of the slot numbers of time slots in an idle state which are linked with pointers is prepared. When a call is generated, a required number of time slots are allocated from the head of the list. When the allocation is canceled, the corresponding time slot number is connected to the end of the list.
When time slots of a time-division multiplex highway are allocated in a fixed manner to transmit cells through an ATM communication line, a maximum number of time slots which can cope with the maximum traffic must be allocated. For this reason, when the traffic of the ATM line is small, the number of time slots which are not used for transmission of cells increases, resulting in a deterioration in the line utilization efficiency of the time-division multiplex highway.
In addition, as disclosed in Japanese Patent Laid-Open No. 4-68730, if band compression is performed in accordance with the kind of information of a cell, the number of time slots can be decreased. If, however, a maximum necessary number of time slots after compression are to be allocated, some time slots are not used either in a small-traffic state. Furthermore, a circuit for performing band compression in accordance is with a kind of information is required. As a result, the arrangement of the ATM interface apparatus is complicated.
According to Japanese Patent Laid-Open No. 2-29196, when a call is generated, time slots required for transmission of cells are allocated. The time-division multiplex highway lines can therefore be efficiently used. However, since a list of time slots in an idle state must be managed, the arrangement of the ATM interface apparatus is complicated.